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Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, Dave Hansen <dave@sr71.net>,
        hpa@zytor.com, tglx@linutronix.de, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, dwmw@amazon.co.uk,
        linux-tip-commits@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
        Arjan van de Ven <arjan@infradead.org>
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From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <97bc2ca2-7d5b-c206-5c23-902ed3586256@linux.intel.com>
Date:   Wed, 14 Feb 2018 18:01:41 -0800
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On 02/14/2018 03:19 PM, Ingo Molnar wrote:
> 
> * Tim Chen <tim.c.chen@linux.intel.com> wrote:
> 
>> On 02/14/2018 12:56 AM, Peter Zijlstra wrote:
>>
>>>
>>> At the very least this must disable and re-enable preemption, such that
>>> we guarantee we inc/dec the same counter. ISTR some firmware calls (EFI)
>>> actually are preemptible so that wouldn't work.
>>>
>>> Further, consider:
>>>
>>> 	this_cpu_inc_return()		// 0->1
>>> 	<NMI>
>>> 		this_cpu_inc_return()	// 1->2
>>> 		call_broken_arse_firmware()
>>> 		this_cpu_dec_return()	// 2->1
>>> 	</NMI>
>>> 	wrmsr(SPEC_CTRL, IBRS);
>>>
>>> 	/* from dodgy firmware crap */
>>>
>>> 	this_cpu_dec_return()		// 1->0
>>> 	wrmsr(SPEC_CTRL, 0);
>>>
>>
>> How about the following patch.
> 
> These fragile complications of the interface should now be unnecessary, as the 
> only driver that called firmware from NMI callbacks (hpwdt.c) is going to remove 
> those firmware callbacks in the near future - solving the problem at the source.
> 
> Thanks,
> 
> 	Ingo
> 

Sounds good. I sent this out before seeing the other mails on removing NMI callbacks
from hpwdt.c

Tim